Some programs do just that. For example Air NZ Airpoints have airpoints dollars as currency, rather than miles. When redeeming on Air NZ ticketed and operated flights, can use airbucks instead of cash (whole fare only, not partially through airbucks) for any available seat. So the award cost automatically varies according to paid fare.

(Note NZ also has fixed amounts awards on partners, on Air NZ special business class awards for elite members, and for Air NZ upgrades.)